The manufacture and fabrication of semiconductor devices involve complex processing steps. During the manufacture of integrated circuits, many layers of different materials are applied to a substrate. These layers overlie one another and must be accurately registered to ensure proper operation of the semiconductor device. If the layers are not properly aligned, the device may not perform well, or may even be inoperative. As semiconductor devices have increased in complexity, the feature dimensions of these devices have decreased, and the influences of optical aberrations become more significant.
To aid in the registration of overlying layers in semiconductor devices, registration patterns, or marks, are included in each layer of the wafer used during fabrication. These patterns have a predetermined relationship when they are correctly registered. A reticle is used to pattern the appropriate marks on a particular wafer process layer, such that the marks can be readily identified by a Registration tool in subsequent processing steps. One example of an alignment mark is a box-in-box mark. An outer box is formed by photolithography, and an inner smaller box is formed in a separate photolithography layering step. When the two boxes are concentric, the layers are accurately registered. Any alignment error produces a displacement of the boxes relative to each other.
Because semiconductor devices are complex and expensive to fabricate, it is desirable to verify registration after the application of each layer. If the displacement of layers is outside of the acceptable limits, defective layers can then be removed and replaced. Registration measurement, verification, and correction is therefore critical to the successful fabrication of these semiconductor devices.
Registration measurement, verification, and correction can be limited by optical aberrations introduced during the photolithography process. Aberration errors are of particular significance given the reduction of sizes of patterns in semiconductor devices. Aberrations affect the ability to accurately measure overlay error. Shift quantity measurements may not correspond to the actual shift quantities.
There are different forms of aberrations that can affect registration verification. Coma aberration exerts the largest influence on the determination of overlay error. Shift of a wave front caused by coma aberration is large at a peripheral portion of a lens and is small at a central portion. Diffracted rays of a large semiconductor pattern are not significantly affected by coma aberration because they have a small diffraction angle and pass through a central region of a lens, causing less wave front aberration. However, a small semiconductor pattern allows passage higher frequency light, which will be more affected by a diffraction phenomenon of a lens. Therefore, the rays diffracted by a small semiconductor pattern have a large diffraction angle, and pass through a peripheral region of a lens, thereby exhibiting more of a coma aberration.
Astigmatism is another optical aberration that occurs because a wave surface in general has double curvature. In this form of aberration, the rays from an object point do not come to a point focus, but rather intersect a set of image planes in a set of ellipses, the diameters of which are proportional to the distances of the two foci from the image plane in consideration.
Spherical aberrations have symmetry of rotation, and are direction-independent. These aberrations occur because rays of different aperture usually do not come to the same focus. These aberrations are also sometimes referred to as aperture aberrations. Spherical aberration occurs in simple refraction at a spherical surface, and is characterized by peripheral and paraxial rays focusing at different points along the axis.
As discussed earlier, semiconductor devices have increased in complexity. The feature dimensions of these devices have decreased, and the influences of overlay errors and optical aberrations have become more significant. It is critical that both overlay errors and optical aberrations be estimated accurately and easily to optimize the critical dimension manufacturing process.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need for improved mechanisms to estimate overlay errors and optical aberrations, such as astigmatism, coma, spherical aberration, and defocus.